Field of the Invention
The present invention generally relates to the field of thin film electronic devices fabricated by additive manufacturing methods. In particular, the present invention is directed to a shadow mask used for patterning materials such as metals, oxides, and OLED compounds.
Description of Related Art
There is currently great interest in additive manufacturing methods for fabrication of thin film devices. Such methods would offer an alternative to established methods such as photolithography. Investigating alternatives to photolithography is of interest because photolithography requires significant facility floor space, requires numerous complex steps per patterned feature, and produces significant toxic chemical waste.
It is known in the art of additive manufacturing to employ the method of shadow masking. Shadow masking involves placing a substrate in close proximity or contact with a shadow mask that includes numerous formed apertures. Subsequently, material is deposited through the apertures, yielding the desired pattern on the substrate. Most often several shadow masks are required to produce a desired thin film device. Thus, it is necessary to precisely align the series of shadow masks with reference points on the substrate.
Typically, thin film devices require features to be accurately located within no more than a few micrometers of their specified location. Current alignment methods are capable of accurately placing the center of the shadow mask within less than 1 μm of its desired location. However, the shadow mask typically contains features over a significant area, hereinafter referred to as the “array.” Therefore, the size of the array must be controlled very precisely if the features of several masks are to be placed within their specified areas.
Because of the desired patterned feature sizes (typically >100 μm), the shadow mask is typically made of a thin metal foil less than 100 μm thick. Such a thin material does not have the structural integrity to be easily and safely handled on its own. Additionally, the shadow mask will be subject to elevated temperature during the deposition process and it is desired that the apertures of the array remain at constant locations during the thermal cycling. Therefore, tensioning the shadow mask and mounting the shadow mask to a rigid frame allows the shadow mask to be easily and safely handled while being robust to temperature changes.
Heretofore, heat was used to tension the shadow mask before it was attached to the frame. In this method, the frame is made of a low coefficient of thermal expansion alloy, such as invar. The frame and shadow mask are heated to a temperature where the array grows, via thermal expansion, to the proper size and an adhesive bonds the shadow mask to the frame. As the assembly cools, the shadow mask contracts more than the frame resulting in a tensioned shadow mask. This method of thermal tensioning does work but it has limitations and requires significant skill of the fabricator.
One limitation of thermal tensioning is that heat causes the shadow mask to expand proportionally in all directions. This method would be sufficient if all pre-mounted shadow masks are dimensionally accurate to the micrometer level. However, shadow masks often do not start with correct proportions. An example of this concept is shown in FIG. 1. In this example, the particular shadow mask requires more stretch in the horizontal direction than in the vertical direction. The best course of action for mounting this shadow mask using thermal tensioning is to select a mounting temperature that splits the difference between the two desired dimensions. This introduces dimensional inaccuracies to the array of this shadow mask which could cause the shadow mask to become unusable. Therefore, the inability to adjust shadow mask dimensions independently is a limitation of thermal stretching.
When shadow masks are mounted via thermal tensioning, the selected mounting temperature should be higher than the maximum temperature realized at the shadow mask during deposition. When this is done the patterned features are generally observed to remain in their starting locations over the course of the thermal cycling of the deposition. According to observations, this starting location is the “cold” or room temperature dimension of the shadow mask. This differs from the mounting, “hot,” dimension of the shadow mask. When the shadow mask cools from the mounting temperature and becomes tensioned, the array changes in dimension. This concept is shown in FIG. 2. This process is somewhat repeatable in that shadow masks with the same aperture pattern tend to deform in the same fashion when transitioning from hot to cold states. However, achieving acceptable shadow mask mounting results requires collecting extensive data on the behavior of shadow masks and significant decision making from the fabricator. Further, transition from hot to cold can result in bowing of the array, a problem that cannot be directly addressed in the thermal tensioning process.
Including an elevated temperature within the mounting process also adds further complications. Introducing an environment of elevated and adjustable temperature greatly complicates the task of making the accurate micrometer scale measurements that are required for a precise shadow mask. Additionally, since it is continually desired to mount shadow masks at increasingly high temperatures, ease of handling and even operator safety become a concern.